The need for energy storage and its rising demand has become a major issue that the world faces today and going forward in the future. For transportation applications, the implementation of Li-ion batteries in Plug-In Hybrid Electric Vehicles (PHEV) and eventually Electric Vehicles (EV) is quickly emerging. The specific energy and power of Li-ion batteries continues to grow as high-performance anode and cathode materials become commercially available. Recent advances in cathode materials have resulted from blending the materials such as layered LiNi0.8Co0.15Al0.05O2 (NCA) and spinel LiMn2O4 phases to create bi-functional behavior: high-rate (spinel) and high-energy (NCA) capabilities. These two advantageous properties are contained in one electrode. However, one would prefer a sole material that can intrinsically provide both high-rate and high-energy.
In 2000/2001, Johnson and Thackeray [1] first published in Proceedings of the International Workshop on Electrochemical Systems, The Electrochemical Society, PV 2000-36, 47-60 (2001) a new ‘layered-layered’ composite material that contained Li2MnO3 with LiNi0.8Co0.2O2 that was integrated together and implemented as a new class of cathode materials that featured acceptable capacity and good stability. Later the ‘inactive’ Li2MnO3 was electrochemically activated at voltages above about 4.5 V, leading to a MnO2 layered phase that was integrated in the structure [2]. Subsequent materials featured Mn and Ni-rich compositions with extra lithium [3]. This process dramatically increased the reversible capacity above about 220 mAh/g. Later work included electrodes designed with ‘layered-spinel’ structures [4].
In an attempt to provide a material with bi-functional behavior, Park et al. [5] synthesized a series of xLi[Mn1.5Ni0.5]O4.(1−x){Li2MnO3.Li(Ni0.5Mn0.5)O2} structurally integrated ‘layered-layered-spinel’ composites, where x was varied from 0 to 1 [5]. These Ni(II)- and Mn(IV)-containing materials achieved high-capacities of 250 mAh/g, due to electrochemical activation of the Li2MnO3 component and high rate capability (200 mAh/g; C/1 rate) due to the Ni-containing layered and spinel components. The ordering of Ni/Mn within the structure was elucidated by Cabana et al. using Li-6 MAS NMR, and high-intensity X-ray diffraction [6].
In regards to layered-only type materials, Kang et al. [7] demonstrated that layered oxide cathodes can yield a very high-capacity of 183 mAh/g at a 6 C rate from an active Li(Ni0.5Mn0.5)O2 material synthesized by lithium ion-exchange from the Na(Ni0.5Mn0.5)O2 precursor made from the co-precipitated Ni0.5Mn0.5(OH)2 starting material. The size mismatch in cationic radii of the Na (1.02 Å) versus Ni2+ (0.69 Å) resulted in less anti-site disorder in the precursor that carried over to the Li-exchanged product. The final reaction to form a Li-containing material was by an ion-exchange reaction with LiBr in hexanol, a method introduced earlier by Armstrong et al. [8] and Capitaine et al. [9] from sodium compounds in their synthesis of layered LiMnO2, and variants thereof [10,11].
Roughly ten years ago, Paulsen and Dahn [12] reported on lithium deficient cathode materials of typical composition Li2/3(Ni1/3Mn2/3)O2 in Solid State Ionics Vol. 126, 3 (1999). These types of materials were made from Li-ion exchange of the starting material layered sodium bronze analogs, which is similar to that used in the present invention. Such materials and variants of these materials where the Ni/Mn ratio was altered or Co was added were also synthesized, characterized and evaluated in lithium cells. The sodium bronzes starting materials were synthesized from single metal hydroxides, oxides and carbonates. However, there was no data in the publication on the addition of extra lithium to the sodium layered bronze that was synthesized as the precursor.
The Paulsen and Dahn work was extended by Eriksson, et al. [13] in Chemistry of Materials, Vol. 15, 4456 (2003) to include compositions Na0.7MnO2 or Na0.7Mn0.89M0.11O2, (where M=Fe, Co, Ni, Cu, Zn, Li, or Al). The sodium starting materials were made by the glycine nitrate process from individual metal nitrates. The sodium reactants were subsequently ion-exchanged with lithium in an organic solvent with lithium bromide. Lithium cells were tested using the Li-ion exchanged compositions as the electroactive material. Using their terminology of O2 and O3, it was concluded that the structure of the Li-exchanged final product was an O2 structure or an intergrowth of O2/O3 with stacking faults. The structure type was dictated by the type of M cation substituted. The electrochemical discharge voltage profile showed one continuous sloping profile and no indication of a phase change to spinel were observed.